Semiconductor devices



June 7, 1960 J. l. PANKOVE SEMICONDUCTOR DEVICES Filed March 19, 1958 fff! mVENToR. Taunus-s l. PBNKQVE- La/maf niteci States Patent OSEMICONDUCTGR DEVICES Jacques I. Pankove, Princeton, NJ., assignor toRadio Corporation of America, a corporation of Delaware Filed Mar. 19,1958, Ser. No. 722,501

18 Claims. (Cl. 317-235) vvbody, such as a `rod or cylinder of germaniumor silicon or the like, having a rectifying electrode disposed be-.tween two non-rectifying or ohmic electrodes. The semiconductor bodymay be of either conductivity-type, but .-is usually N-type, sinceelectron mobility is greater than .hole mobility. One of the ohmicelectrodes is known as the source, while the other is known as thedrain. The electron current of the device ows from the source to thedrain electrode. The current conducting portion of the semiconductorbody between these two electrodes .is known as the channel. Therectifying electrode, which vserves as the input electrode, is known asthe gate. In operation, the rectifying electrode is biased in thereverse direction, so vas to extend a depletion layer intothesemiconductor body and thus increase the resistivity of the.conducting channel between the source and the drain. The input signalis impressed on the rectifying electrode, vand modulates the depth orthickness of the depletion xlayer. The resistance of the semiconductorbody is .thereby varied by the inputrsignal, and hence the outputcurrent is modulated by the signal current. However, .-it has hithertobeen dii'cult to produce satisfactory uni- Apolar devices forhigh-frequency operation.

Accordingly, an object of this invention is to provide improvedsemiconductor devices.

'Another object of this invention is to provide improved unipolardevices.

Still another object of this invention is to provide improved unipolartransistors suitable for high-frequency operation An additional objectis to provide a simpliiied and in- :expensive method of making unipolardevices.

These and other objects of the instant invention are accomplished byproviding a device comprising Ia semiselected is one which is capable ofimparting said given -..conductivity type to the semiconductor body,hence the .contact between the metal band and the opposite conductivitytype surface zone is rectifying in character.

4At two opposite ends of the body there is connected an electrode thatis ohmic or non-rectifying with respect to Vthe thin surface zone. Tocomplete the device, electrical Yleads are connected to each electrodeand to the metallic ring, which serves as the gate electrode. Inoperation,

2,940,022 Patented June 7, 1960 the thin opposite conductivity typesurface zone of the body serves as the conducting channel between thesource and the drain, while the given type central zone or core of thebody is electrically inactive.

The invention will be described in greater detail by reference to thedrawing, in which:

Figures la-ld are cross-sectional views illustrating successive steps inone method for the fabrication of a device in accordance with theinvention;

Figure 2 is a section along the line 2-2 of Figure 1b; and

Figure 3 is a schematic diagram of a unipolar device in accordance withthe invention and an associated circuit for its operation.

Similar reference characters are applied to similar elements throughoutthe drawing.

Referring to Figure la, a semiconductive body 10 is prepared of givenconductivity type monocrystalline material such as germanium or siliconor the like. In this example, the body 10 consists of a germanium rod orcylinder containing sufficient donor impurity atoms to be of N-typeconductivity. The donor may for example be antimony. The semiconductorbody is then converted to opposite type conductivity by adding afast-diiusing acceptor impurity. In this example, the body 10 isimmersed in an aqueous solution of copper nitrate for a few seconds,then heated at 750 C. for l0 minutes in an inert atmosphere such asargon. During this step copper atoms diffuse through the body 10 andconvert it to P-type.

Referring to Figure 1b, a metallic ring 11 is fused around thesemiconductor body 1li. The ring =11 is made of a metal or alloy whichinduces in the particular semiconductor utilized conductivity of thesame type as the fast-diffusing impurity.- -In this example, themetallic ring or band 11 consists of indium. The indium ring 11 and thegermanium body 10 are heated at about 550 C. for about l0 minutes in aninert atmosphere. Since copper is more soluble in indium than ingermanium, the copper atoms in the surface re'gion 12 adjacent to theindium ring 11 diffuses into the indium, and remain dissolved therein.The process involved is similar to the extraction of a solute from aiirst solvent by means of a second solvent which has a greatersolubility for the solute.

Although the surface zone ,12 is depleted with respect to copper atoms,the concentration of donor impurity atoms in zone 12 remains virtuallyconstant, since relatively few donor impurity atoms diiuse into theindium. It has been found that the diiusion constant of copper ingermanium is about one million times greater than the diffusion constantof conventional donor impurities such as phosphorus, arsenic andantimony. Since the acceptor concentration of the surface region 12 isconsiderably reduced while the donor concentration remains virtuallyconstant, the surface zone 12 is reconverted to N-type conductivity. APN junction is thus formed at the interface 14 between the N-typesurface zone 12and the P-type central zone 13 or core of thesemiconductor body 1t). A second PN junction 15 is formed between theindium ring 11 and the N-type surface zone 112. Figure 2 is a sectionalong the line 2-2 of Figure lb.

Referring to Figure 1c, an electrode 16 is attached to one end of thesemiconductor body 10. The electrode 16 may for example be a pellet or adisc, and may be soldered or alloyed to the semiconductor body 10.However, the electrode material and any solder utilized must make anohmic contact to the N-type surface zone 12, and hence a rectifyingcontact to the P-type core 13 of the semiconductor body 10. In thisexample, the electrode 16 is a disc composed of 99% lead-1% arsenic, andis soldered to the semiconductor body. The arsenic present makes theelectrode strongly Ntype, thus insuring an ohmic connection to theN-type surface zone 12 and a rectifying junction 19 with the P-type core13. A similar electrode 16 is attached to the opposite end of theVsemiconductor body, forming an ohmic connec- Vtionjwith surface*zone-12 and a rectifying junction 19' with Ithe P-type core 13.Referring to-Figure ld, the device is completed by attaching leads 17and 17'` to electrodes 16 and 16 respectively, and Vlead 18 tothe'indium ring 11. The leads may for examplebe copper, nickel or one ofthe noble metals. 'i

Referring to Figure 3, therdevice shown inY Figure ild may beoperatedjin a circuit utilizing electrodes 16 and.

16 asthe source and drain electrodes respectively., In the operation ofa device having an N-type channel' and Ia P-type gateregion, thenegativeterminal of asuitable 'power supply such as'a' battery 31 is connected.to the source electrodelead 17V. The positive terminal of the Y ttery31 is connected Vtothe loadcircuit 32, which in turn is connected to the'drain'electrode lead117. The control or gate electrode 11 is .connectedby means of gate lead 18 to a signal generator 33 andV to the negativeterminalH of a bias Ibattery 34. The positive termii -nal of the biasbattery 34 iszgrounded. "The above polarities are reversed Vfor a devicehaving'a P-type channel and an N-'type gate. Y

In the mode of operation of the device shown in Figure f3, the N-typesource electrode 16 injects a current of Velectronsrinto the N-typesurface zone 12. Electrons are not injected into the P-type core, sincethe PN barrie'r between the arsenic-.doped N-type source 16 and theP-type core 1-3 opposes such a ow. The electron 2O Y o alternatively Vhefabricated-as follows.

vvarious `electrodes are reversed, the bias nel is a thin P-type zoneand around an N-type core. In such devices, the kcurrent which flows inthe P-type channel between the source and drain consists of holes. Forexampie, a rnonocr/stalline` germanium cylinder may be prepared bymethods known to theart with sufiicient indium as impurity to be. ofP-type conductivity. Lithium is did-used into the germanium to convertit to N-type conductivity. An annular electrode of 99% lead- 1%arsenic'is then alloyed around the germaniumrcylinder, thereby leachingsucient lithium from the surface zone of the cylinder to leavesaid zoneP-type. The annular electrode contains arsenic donor atoms, hence itforms a rectifying contact to the P-type surface zone. Iridiumelectrodes are next alloyed toV each end of the cylinder. Theseelectrodes are'ohmic Yto the P-type surface zone, but rectit'ying to theN-type core. It will be understood that when the conductivity types ofthe voltageV polarities are alsoreversed as required. p I Y 'Y Unipolardevices in 'accordance with die invention may A Vgiven conductivitytypeY monccrystalline Wafer is prepared ofV a `Ycurrent'ows through theN-type zone 12, which serves as thecurrent channel, to the drainelectrode 16'. The gate electrode 11, by means of the bias voltage ofbias battery 34 and the signal'voltage of signal generator 33 "appliedthereto, forms a `depletion layer which extends --from the rectifyingvbarrier. 15 into the conducting channel Vof the deviceYanclgn'toditiesl the resistivity of the thin surface-'zone' 12 whichacts as the channelY between the f'source 16 and the drain 16.4 The gateVelectrode thus 'modulated inV laccordance with the applied signal, theg' outputcurrent'owing Ibetween the source Vand drainY elec-'trodesrthrough the Vload 32. I

A-'feature of unipolar devices according to thisinvenftion is-thecombination of a thinchannel and a gate havving-a large girthto lengthratio, which-'results in'greater,V

sensitivity jand improved performance at higher fre- -quencies than; inconventional vunipolrar devices. A-An'other-"feature of unipolarvdevices in `accordance with nthis invention isfthe provision of acentral coreof given conductivity type Yelectrically inactive materialaround vvwhichfis disposed a thin electrically active zone of oppo- Ysite conductivity type; Although'the central core ofthe :device does nottake an active part .in `the electrical operation thereof, the coremakes rthe device more rugged v-and Vveasier to handle. Anotheradvantage of this struc- '.tur'efis that itV Venables easyvr attachmentof the ohrnic {source and drain electrodes at each end of the unit.

- In larger units intended for. operation at high power' levels, a holemay be boredAthrough-the central core 13, and a ycoolant lcirculatedtherethrough to remove the heat :dissipated by the unit, iri a'mannersimilar to that de- -rscribed in my Patent 2,754,455J issued Iuly l0,1956, YandV assignedto .thesarne assignee. The power handling capabilityofrthe unit is thereby increased. V ,It'will be understood Vthat in thedevice described the '-rnat'erials'have been mentioned by wayof exampleonly.

' vOthercombinations vof acceptors and donors, suchas nickel andarsenic, `rnay'be utilized. The invention may Y Y falso be practiced.with fother monocrystalline semiconductors-such asosilicon,germanium-silicon alloys, indium Yphosphide, gallium arsenide, and thelike;

For VspecialV applications,'tl1e conductivity types may be r "reversedto form devices in which'the conducting chan-y Vnitrogen containing adonor impurity, whichmay for V semiconductive material such as gallium,arsenide or sili- 'con or 'thelike 25 The Wafer may be a plate oracylinder ora generally rod-shaped body. V'In this example, the waferconsists of a silicon cylinder containing sufcient acceptor impurityVatomsV to be of P-type conductivity and about 50olnn-centinret'erYresistivity. The

' acceptor lmay for example be boron.

Next a thin surface zone of `the silicon Ywafer is converted .toopposite conductivity type. When the `wafer is P-t'ypfe, a surface zoneVof N-type conductivity lmay be formed by heating the wafer in anVatmosphere of example be phosphorus. In thisl example, the nitrogen haspreviously been passedfoverl phosphorus pentoxide kept at about 220? C.to 660'? C. An amorphous glassy phosphorus-containing film Vis V-formedover the rWafer surface. Therarnbient is then changed to pure nitrogen,and the wafer is heated for about 1/2 hour at-about 1300 C. YDuring thisstep the Vphosphorus diiusesrfrom vthe glassy surface lm Vinto ItheadjacentV Azone of the wafer, and Yforms a thin N-typeY layerY over thesurface of the silicon body.` .In this-example, the N-type surlface zonethus produced is about 0.5 thick.Y f

Nextan annular gatefelectrode is secured to the silicon'cylinderingrectifyingY Contact to the surface zone thereof. In this example,the'annular electrodeconsists of an vindium ring, whichisealloyed aroundthe silicon cylinder byheating theY assembly of Wafer andV ring to Yatemperature Vof about-,300"4 VVC. forV about l0 minutes. Under theseconditions,the depth of penetration by the Vringl into the waferV isshallow, which is 1 desirablesince the N-type surface zone isVrelatively thin. fV

Electrode pellets yare then attached to each end-of the Vsiliconcylinder by either alloying or solderingtechniques.

These electrodes serve las the source and the drain of the unit. Theelectrode pellet material is one -whichforms a high conductivitynon-rectifying electrical contact with the N-type surface Vzone of thesilicon body, and a recti- YYfying` contact with the P-type core orcentral zone of the body. vOne suitableelectrode composition consists ofparts lead and Y12 parts antimony, yas disclosed in application SerialNo. 309,867, assigned Yto the same` assignee.

YAnother suitable electrode'composition consisting of lead,

NYgold, and a donory is disclosed in U.S. application Serial No.433,351, Iassigned to the same jassignee. A tluxcon-V sisting of afluorine salt may be utilized to promote the Valloying of the pellet tothe silicon wafer, as vdescribed 'September 24, 71957, assigned in U.S.2,807,561 issued To complete the.dev1ce,leads are attached to theelectrode pellet at each end of the unit, |and to the annular nickel, orVa noble-metal, c i

J LJ actionn' t beuunrderstood that this irlethod` can practiced withall the conventional monocrystalline semiconductor materials, includingsemiconductive compoundscon devices may be operated at higher, ambienttempera! tures than ygermanium devices, and gallium arsenide ldevicesare capable of operation at higher temperatures than silicon devices.

It will also be understood that the conductivity types of the variouszones of the device ldescribed, may .be

reversed, providing that the polarity ofL the applied bias voltages -issimilarly reversed as required.Y y

There have thus been described new and useful forms of semiconductordevices, as well las methods for making these devices.

What is claimed is:

1. A semiconductor device comprising ya monocrystalline semiconductivebody having a central zone of given conductivity type and ya thinsurface zone of opposite conductivity type, -a metallic ring around saidbody, said metal being capable of imparting said given conductivity typeto said semiconductongan electrode connected to each end of said body,said electrodes being ohmic with respect to said surface zone, and leadsconnected to each said electrode and said metal ring. v

2. An electrical device comprising a monocystalline semiconductor bodyhaving a. central zone of given conductivity type and a thin surfacezone of opposite conductivity type, a metal band in rectifying contactto said surface zone around said body, an electrode connected to eachend of said body, said electrodes being nonrectifying with respect tosaid surface zone, and leads connected to each said electrode and saidmetal band.

3. An electrical device comprising a rod-shaped monocrystallinesemiconductor body having -a central zone of given conductivity type anda thin surface zone of opposite conductivity type, a metallic ring`alloyed around said body, said ring being in rectifying contact withsaid surface zone, an electrode connected to each end of said body, saidelectrodes being non-rectifying with respect to said surface zone, andleads connected to each said electrode and said metal ring.

4. An electrical device comprising a rod-shaped monocrystallinesemiconductor body having a central zone of given conductivity type anda thin surface zone of opposite conductivity type, a metal band alloyedaround said body, said metal being capable of imparting said givenconductivity type to said semiconductor, an electrode connected to eachend of said body, said electrodes being non-rectifying with respect tosaid opposite type surface zone, and leads connected to each saidelectrode and said metal band.

5. An electrical device comprising a rod-shaped monocrystallinesemiconductor wafer containing two typedetermining impurities, thecentral zone of said wafer containing an excess of one said impurity soas to be of given conductivity type, the surface zone of said Wafercontaining an excess of the other said impurity so as to be of oppositeconductivity type, an annular electrode wrapped around said wafer inrectifying contact with' said surface zone, an electrode connected toeach end of said Wafer in non-rectifying contact to said surface zone,and leads connected to each said electrode.

6. An electrical device comprising a rod-shaped monocrystallinesemiconductor wafer containing both acceptor and donor impurities, thesurface zone of said wafer containing -an excess of said donorimpurities so as to be of N-conductivity type, the central zone of saidwafer containing an excess of said acceptor impurities so as to be ofP-conductivity type, an annular electrode wrapped around said wafer andin rectifying contact with said N-type surface zone, an electrodeconnected to each end 6 Y Y ofl said wafer in'fnon-rectifying contact tosaid `surface7 zone, and leads connected to each said electrode.

7. A Yunipolar transistor. comprising a rod-shaped monocrystallinesemiconduc'tive germanium Wafer having both acceptor and donorimpuritiessaid acceptor4 impurities 4consisting principally of nickelatoins and said" donor impurity consisting ll'iri'ncipally of arsenic`Iatoms; the surface zone of said wafer containing an excess of thecentral zoneof said wafer containing" an excess of said nickel atoms Ysoas to be of P-conductivity type, annular indium electrode alloyed aroundsaid wafer in.

rectifying contact with'said'N-type surface zone, an elec-z trodealloyedtoeach end of said wafer innen-rectify#- ing contact to said surfacezone, ,and leads connected toJ each said electrode. f A 8. A unipolrtransistor comprisinga monocrystallline"V semiconductive germaniumcylinder having vboth acceptor and donor impurities, saidacceptor-,impurity consisting principally of copper vatoms and saiddonorimpurity con?y sisting principallyof antimony atoms, the surfacezone of said'cylinder containing excess of .said .antimony atomsso as tobe of `Nconductivity type, the central zoneV of said cylinder containingan excess ofrsaidcopper atoms,` so as to be of P-conductiv-ity type, 'anannularindium electrode alloyed around 'said' 'cylinderxin 4rectifyingcontact with said Ntype surface zone, anfelectrode alloyed to each Yendof said' cylinder in Vnim-'rectifying contact` to said surface zone,1and leads connected-to each said electrode. l' Y Y 9. A unipolartransistor comprising'a monocrystalline semiconductive silicon cylindercontaining both acceptor and donor impurities, said acceptor impuritiesconsisting principally of boron atoms and said donor impurity consistingprincipally of phosphorus atoms, the surface zone of said cylindercontaining an excess of said phosphorus atoms so as to be of`\Iconductivity type, the central zone of said cylinder containing anexcess of said boron atoms so as to be of P-conductivity type, anannular indium electrode alloyed around said' cylinder in rectifyingcontact With said N-type surface zone, anv electrode alloyed to each endof said cylinder in non-rectifying contact to said surface zone, andleads connected to each said electrode.

10. An electrical device comprising a rod-shaped monocrystallinesemiconductor wafer containing both ac-V ceptor and donor impurities,the surface zone of said wafer containing an excess of said acceptorimpurity so as to be of P-conductivity type, the central zone of said.wafer containing an excess of said donor impurities so as.. to -be ofN-conductivity type, an annular electrode,.- wrapped around said waferin rectifying contact with.`

said P-type surface zone, an electrode connected to cachaend of saidwafer in non-rectifying contact to said sur` face zone, and leadsconnected to each said electrode.

11. A unipolar transistor comprising a monocrystalline semiconductivegermanium cylinder containing both acceptor and donor impurities, saidacceptor impurity consisting principally of indium atoms and said donorim. purity consisting principally of lithium atoms, the surface zone ofsaid cylinder containing an excess of said indium atoms so as to be ofP-conductivity type, the central zone of said cylinder containinganexcess of said lithium atoms so as to be of N-conductivity type,` anannular electrode of 99% lead-1% arsenic alloyed around said cylinder inrectifying contact with said P-type surface zone, an electrode Ialloyedto each end of said cylinder in non-rectifying contact to said surfacezone, and leads connected to each said electrode.

l2. A method of fabricating a unipolar transistor cornprising the stepsof,.doubledoping a rod-shaped monocrystalline semiconductor wafer with afast-diffusing impurity of given conductivity type, and a slow-diffusingimpurity of opposite conductivity type, alloying around said wafer anannular metal electrode so as to leach a ducir mony, aid fast-diEuSingimpurity Mimo@ iigiflsjinfwhch, Sgam 'seiniconf said, s1Qw'-diiusing,.impurity is in diumlsiqjismffusmg impurity isf-lithium( and said

